Depression is a debilitating and common medical condition that costs billions of dollars in healthcare costs and lost productivity in the United States each year, according to the National Institute of Mental Health (NIMH). People experiencing depression feel hopeless, worthless and experience a loss of interest in everyday activities such as work, hobbies or sex.

Nearly 25 percent of the adult population suffers from depression at some point. Many factors can trigger depression such as: serious illness, divorce or the death of a loved one. Sometimes there is no apparent reason for depression.

Alone or in conjunction with therapy, antidepressant medications are a widely used, effective treatment for depression. More than 80 percent of people with depression improve when they receive appropriate treatment, according to the NIMH.

The brain communicates thoughts and feelings through the use of special chemicals that send messages. Two important chemical messengers, called neurotransmitters, are serotonin and norepinephrine.

There is a strong connection between the amount of these chemicals in the brain and a person's mood. If levels of serotonin and norepinephrine get too low people usually feel depressed. Drugs can help elevate these brain chemicals.

The newer antidepressant medications, such as the selective serotonin reuptake inhibitors (SSRIs), are popular because they have fewer side effects than the older medications, which include tricyclic antidepressants (TCAs) and monoamine oxidase inhibitors (MAOIs). Although both generations of drugs are effective in relieving depression, some people will respond to one type of drug, but not another.

Many antidepressant medications can have drug interactions with other prescription and over-the-counter medications, herbs, alcohol, and even foods. You should consult with your doctor or pharmacist prior to starting treatment to find out about mixing them with other substances and other important precautions.

The most common types of antidepressants are:

SSRIs (Selective Serotonin Reuptake Inhibitors)

These drugs increase the brain's level of serotonin to improve mood. They make serotonin more available to the receiving nerve by preventing (inhibiting) the return (reuptake) of the chemical serotonin to the sending nerve. SSRIs have also been shown to be useful in the treatment of depression, obsessive-compulsive disorder (OCD) and some forms of severe shyness.

They generally have fewer bothersome side effects than other types of antidepressants and are effective for many people. Some common SSRI side effects include: loss of appetite, heartburn, drowsiness and sexual difficulty.

Common SSRIs include, Celexa (generic name citalopram), Luvox ( fluvoxamine), Paxil (paroxetine), Prozac ( fluoxetine), and Zoloft (sertraline).

Tricyclic antidepressants are very effective in combating depression, but can cause troublesome side effects such as drowsiness, dry mouth and constipation. Common tricyclics include, amitriptyline, desipramine, nortriptyline, and Remeron (mirtazepine).

MAOIs (monoamine oxidase inhibitors) are another common class of antidepressant drugs. Because they can have potentially life-threatening drug and food interactions, MAOIs are rarely prescribed except for depressed people who haven't improved with other antidepressants. If you are taking a MOAI drug, consult with your doctor before you take any other medicines. Your doctor will also tell you which foods to avoid mixing with your medicine. MAOIs include Nardil (phenelzine), and Parnate (tranylcypromine).

Miscellaneous antidepressants:

Other commonly prescribed antidepressants include: Effexor (venlafaxine), a drug that works by inhibiting the uptake of both serotonin and norepinephrine, Serzone (nefazodone), trazodone, and Wellbutrin (bupropion).

Medications work differently for every person. Many people begin to feel the effects of medicine even in the first few weeks of treatment. After about six weeks, more than half of the people who begin taking antidepressant medicine will feel more like their usual self.

It is important for someone with depression to continue taking their medication, even if symptoms do not improve immediately or they feel too discouraged to stick with their treatment. If symptoms do not improve over time, a change in medication may be necessary.

At the beginning of treatment, your doctor will want to see you more often (possibly every week). The purpose of these visits is to check the dosage (how much and how often you take the medicine), to watch for side effects (problems caused by the medicine), and to see how well the treatment is working on relieving symptoms of depression. Once you begin to feel better, you probably will visit the doctor less often.

If you are taking antidepressant medications, you must keep all of your appointments whether you are feeling better or worse so your doctor can check your progress and watch for side effects. Keeping a record of your symptoms is a helpful way to monitor your progress.

Side effects

Up to half of those taking antidepressants have some side effects early in treatment (in the first 4 to 6 weeks); side effects are usually not a problem after that. For a small number of people, side effects are bad enough to require that their doctor have them stop the medicine.

Even though every antidepressant can have some side effects, not everyone gets them, and some have different side effects than others. Let your doctor know about any side effects or difficulties that you may be having with medications. Whether you are having problems or feeling better with antidepressants, it is important that you don't stop your medication without talking to your doctor first. Some medications need to be tapered off gradually. If you stop medications too soon, you increase the chance of symptoms returning because of a relapse of depression.